Communication networks are targets to different threats such as intrusion, viruses, trojan horses, worms, spyware, adware, denial of service attacks, distributed denial of service attacks, teardrops attacks, TCP SYN (transmission control protocol synchronization) attacks, one on one attacks, dictionary attacks, unauthorized access, unauthorized usage of VoIP (voice over Internet protocol) systems and VoIP eavesdropping. Threats to a network may originate from both outside the network and inside the network.
Commonly used measures against such threats include perimeter firewalls, personal application firewalls (PAF), internal security gateways (ISG), host-based security software and anti-virus software.